Processes for forming a thin film on an article, such as a spectacle lens, are well known. Thin films having a uniform distribution of optical properties can be automatically produced in large quantities with excellent stability and reproducibility by controlling the amount of the film-forming material “in flight” (i.e., vaporized) during production. The amount of film forming material in flight during production is controlled by comparing a specific measured optical parameter value, which depends uniquely on the optical film thickness, to a predetermined reference value for the measurement. During thin film formation, the measured parameter's value changes continuously during the process of thin film formation and the system or apparatus for forming the thin film makes adjustments until the measured parameter value agrees with the reference value for the measurement.
For example, as disclosed in Japanese Patent Application Laid-Open No. 2001-115260, a process for forming a thin film utilizes the light transmission and reflection characteristics of an article having a thin film formed thereon. When an article having a thin film formed thereon is irradiated with prescribed light, the amount of light transmitted through, or reflected from, the thin film depends on the refractive index and the thickness of the thin film. Using these optical properties of thin films, an optical film thickness meter is used for measuring the amount of light transmitted by, or reflected from, the thin film. Subsequently, the system for forming the thin film is operated to continuously control the amount of film-forming material in flight until the amount of transmitted light or reflected light measured by the optical film thickness meter is close to, or the same as, a reference amount of light corresponding to the desired thin film thickness.
As further described in Japanese Patent Application Laid-Open No. 2001-115260, there is a problem obtaining exactly the amount of reference light corresponding to the desired thin film thickness. Specifically, the reference amount of light may be maintained by supplying a constant electric power to the electric gun that generates the reference light, which occurs when various operating conditions of the apparatus are optimal. However, more frequently than not, various operating conditions for the system are not optimal and the apparatus for forming the thin film must be controlled using the experience and the intuition of a skilled operator. This makes control of the thin film formation process complicated.
For example, when forming an antireflection film on a spectacle lens for commercial applications, it is often necessary that both the antireflection property and the color of light reflected from the antireflection film be taken into consideration because some customer's want colored spectacles from the standpoint of fashion. To provide specifically colored spectacle lens having an antireflection film formed thereon, a specific process is performed. First, the desired color is set at a wavelength selected by an engineer who sets the interference color. Second, the film is actually formed at the set wavelength and the color of the reflected light is confirmed by the naked eyes. When correction in the color of the reflected light is necessary, the wavelength selected, (i.e., the filter set at the optical film thickness meter), is changed and the above process is laboriously conducted again. This process of forming the antireflection film is repeated until the desired color of the reflected light is achieved. In this case where the process of forming the antireflection film is repeated until the desired color of reflected light is achieved, it is necessary to collect data on the amount of reference light present when the desired color of the reflected light is selected. Thus, markedly complicated trial-and-error operations must be repeated to provide guidance or constructive feedback with respect to achieving the desired color of reflected light during the process of forming a thin film.
Moreover, the types of commercially provided antireflection films are numerous. Since the set or selected wavelengths for these antireflection films are different from each other in many cases, the filters in the optical film thickness meters are frequently different from each other as well. Consequently, this leads to having different apparatuses for forming a thin film in a production line. When a specific antireflection film is formed in all apparatuses for forming a thin film in the production line, the possibility exists that complicated operations must be conducted for actually measuring data of the reference amount of light best suited to the filter in each optical film thickness meter.
The present invention endeavors to overcome the above problems and has as an object to provide a process for forming a thin film, wherein when the wavelength corresponding to the thin film previously formed is changed, the data corresponding to the reference amount of light can be easily measured even though the type of the filter in the optical film thickness meter is different among each of the apparatuses for forming a thin film in the production line. Furthermore, the present invention endeavors to provide an apparatus for practicing the process for forming a thin film of the present invention.